This invention relates to nuclear magnetic resonance (NMR) techniques. In particular, this invention relates to a method for correcting distortions in NMR images due to nonuniformity of magnetic field gradients.
NMR has been developed into an imaging modality utilized to obtain images of internal anatomical features of humans, for example. Such images depicting tissue-related parameters, such as nuclear spin distribution, spin-lattice (T.sub.1), and/or spin-spin (T.sub.2) relaxation constants are believed to be of medical diagnostic value in determining the state of health of tissue in the region examined. In the course of an examination, the patient region of interest is positioned in a substantially uniform polarizing magnetic field produced by one of known means, such as resistive, superconductive, or permanent magnets. Imaging data for reconstructing NMR images is collected by subjecting the region to pulse sequences comprised of magnetic field gradient and RF pulses. The magnetic gradient field and radio-frequency fields are generated by coil assemblies positioned in the polarizing magnetic field and have a generally cylindrical configuration to accommodate the patient region to be studied.
Magnetic field gradients are used in combination with the RF pulses to encode spatial information into NMR signals emanating from the region being examined. Typically, three magnetic field gradients are used, each producing a field whose component parallel to the polarizing magnetic field exhibits a variation with the X, Y, or Z position within a Cartesian coordinate system. The gradients are required to be substantially constant throughout the region of interest, although their magnitudes are typically time dependent. Non-uniform magnetic field gradients in the region to be imaged result in a non-linearity in the space-frequency relationship that exists in the region subjected to a magnetic field gradient. The relationship between field strength and resonant frequency is defined by the well-known Larmor equation, in accordance with which the nuclear spin resonant frequency .omega. is proportional to the applied magnetic field. The resonant frequency for different NMR isotopes is defined by means of a constant known as the gyromagnetic ratio. The linear relationship between field strength and resonant frequency forms the basis of NMR imaging.
Aberrations in the gradient uniformity cause geometric distortion of the image. In one specific example of an NMR imaging technique known as spin-warp imaging (which is a specific example of the Fourier transform NMR technique), the distortions are most apparent near the edges of the field of view, due to the reduction in the gradient strengths there. One solution to the image distortion problem, regardless of which NMR imaging technique is utilized, is to construct the gradient coils large enough relative to the field of view so that the central region has sufficient uniformity for the desired geometric fidelity. One penalty for this approach, however, is that the bore of not only the gradient coils but also of the main magnet producing a polarizing magnetic field must be increased. However the bore diameter can only be increased at great expense, particularly in superconductive magnets which are frequently utilized in NMR imaging. A second penalty is that the power required to drive the gradient coils increases as diameter to the fifth power. Thus, increasing the diameter in order to expand the uniform volume is not an attractive solution. A second approach is to make the gradient coils with higher order correction current distributions in order to increase the useful radius. This solution, however, also requires increased gradient drive power due to additional coil inductance, and entails considerable complication in the coil construction. It is, therefore, a principal object of the present invention to provide a method to render the current gradient coils more suitable for use in NMR imaging.